The present invention relates to a laser radar device, and more particularly to a coherent laser radar device using a pulsed laser that oscillates at a single wavelength as a light source for the purpose of measuring physical information such as a distance, a velocity, a density distribution or a velocity distribution of a target.
A coherent laser radar device using a laser beam can measure a wind velocity or a wind velocity distribution even in fine weather because a sufficient scattering intensity is obtained even through aerosol existing in the atmospheres. For that reason, the coherent laser radar device is preferably located at an airport or mounted on an aircraft and expected as a device for detecting a hindrance including an air turbulence.
The coherent laser radar devices are of two types one of which employs a pulsed laser that oscillates at a single frequency as a light source and the other of which employs a CW (continuous wave) laser.
FIG. 9 is a structural diagram showing a laser radar device in which a coherent laser radar device using an injection seeding pulsed laser device as a light source as disclosed in U.S. Pat. No. 5,237,331 B by Sammy W. Henderson et al. is combined with a wavelength synchronizing circuit for stabilizing the wavelength of the laser radar light source as disclosed in JP 10-54760 A by Shoji and Hirano.
The laser radar device shown in FIG. 9 includes a CW laser light source 1 that oscillates at a single frequency, a first optical divider 2 that branches a laser beam 19 from the CW laser light source 1, a frequency shifter 3, an injection seeding pulsed laser 4, a beam splitter 5, a xc2xc wavelength plate 6, a telescope 7, a scanning optical system 8, a first optical coupler 9, a photo detecting portion 10, a second optical divider 11, a third optical divider 12, a second optical coupler 13, a signal processing device 16, an adjusting mechanism 17 for a cavity length of the injection seeding pulsed laser 4, and a control circuit 18 for the adjusting mechanism. Reference numeral 20 denotes a seed light from the frequency shifter 3, 21 is a pulsed laser beam from the injection seeding pulsed laser 4, 22 is an optical axis of a transmit/reception light, 23 is a transmission light, 24 is a reception light, 25 is a local light and 26 is a coupled light of the reception light 24 and the local light 25 due to the first optical coupler 9.
Subsequently, the operation of the laser radar device shown in FIG. 9 will be described. The laser beam 19 from the laser light source 1 that oscillates at a single frequency f0 is branched by the first optical divider 2 into two beams one of which forms the local light 25, and the other of which becomes a laser beam that increases in frequency by a frequency fIF by the frequency shifter 3 and is then supplied to the injection seeding pulsed laser 4 as the seed light 20.
The injection seeding pulsed laser 4 conducts the pulse oscillation at the single frequency (single wavelength) in an axis mode having a frequency closest to the seed light 20. The laser pulse 21 from the injection seeding pulsed laser 4 which is linearly polarized is reflected by the beam splitter 5 through the second optical divider 11. Thereafter, the reflected light is transformed into a circularly polarized light by the xc2xc wavelength plate 6 and then irradiated toward a target through the telescope 7 and the scanning optical system 8 as the transmission light 23.
A scattered light from the target is received through a backward path of the transmission light. The reception light 24 becomes a linearly polarized light shifted from a polarization plane of the laser pulse 21 by 90 degrees due to the xc2xc wavelength plate 6 and is then transmitted through the beam splitter 5 so as to be guided to the first optical coupler 9. In the first optical coupler 9, the reception light 24 and the local light 25 are coupled together, and the coupled light 26 is supplied to the photo detecting portion 10.
In this example, the photo detecting portion 10 is structured as shown in FIG. 10.
As shown in FIG. 10, the photo detecting portion 10 includes a first photo detector 27 and a second photo detector 28. Each of the first and second photo detectors 27 and 28 is made up of a photodiode that functions as a square-law detector which conducts light coherent detection and a microwave amplifier that electrically amplifies a signal from the photodiode. The microwave amplifier is shown by the combination of a pre-amplifier and a post-amplifier in the figure. A detection output from the first photo detector 27 is outputted to the signal processing device 16 as a reception signal, and a detection output from the second photo detector 28 is outputted to the signal processing device 16 as a monitor signal.
Returning to FIG. 9, the coupled light 26 from the first optical coupler 9 is coherent-detected by the first photo detector 27 of the photo detecting portion 10. A signal from the first photo detector 27 is inputted to the signal processing device 16 as the reception signal. The signal processing device 16 calculates a distance to the target in accordance with an arrival period of time of the reception signal (a period of time since the transmission of the transmission light to the target till the reception of the reception light from the target), analyzes the frequency of the reception signal to obtain a Doppler signal, and extracts the velocity of the target from the Doppler signal.
As described above, the injection seeding pulsed laser 4 is required to monitor a difference in frequency between the pulsed laser beam 21 and the local light 25 in order to obtain an accurate Doppler signal since the injection seeding pulsed laser 4 conducts the pulse oscillation at the single frequency in an axis mode having a frequency closest to the seed light 20. For that reason, after a part of the laser pulse 21 and a part of the local light 25 are extracted as monitor lights from the second and third optical dividers 11 and 12, respectively, and then coupled together by the second optical coupler 13, the coherent detection is conducted by the second photo detector 28 within the photo detecting portion 10a. A signal from the second detector 28 becomes the monitor signal.
In the signal processing device 16, a frequency difference (the frequency of the monitor signal) fM between the laser pulse 21 and the local light 25 and the oscillation timing of the laser pulse are obtained from the monitor signal. Assuming that the frequency of the local light 25 is f0, the respective frequencies fs, fT, fR, fM and fsig of the seed light, the laser pulse, the reception light, the monitor signal and the reception signal are represented by the following expressions.
fs=f0+fIF
fT=fs+xcex94f
fR=fT+fd
fM=fIF+xcex94f
fsig=fM+fd
where xcex94f is a frequency difference between the laser pulse 21 and the seed light 20, and fd is a Doppler frequency of the target. A difference between the frequency fsig of the reception signal and the frequency fM of the monitor signal is taken, thereby being capable of obtaining the Doppler frequency fd of the target.
In order that the injection seeding pulsed laser 4 stably obtains the injection seeding operation, the injection seeding pulsed laser 4 adjusts the cavity length of the pulsed laser by using a piezoelectric element as the adjusting mechanism 17 for the cavity length. The piezoelectric element that functions as the adjusting mechanism 17 of the cavity length is controlled by the control circuit 18. In the signal processing device 16, an error signal based on a value of the frequency difference fM between the laser pulse 21 and the local light 25 is transmitted to the control circuit 18 from the monitor signal. In the control circuit 18, the cavity length of the pulsed laser 4 is adjusted by the piezoelectric element so that the value of xcex94f is set to be a set value or less, or 0.
In this way, the laser pulse that stably oscillates in a single mode (single wavelength) is obtained.
FIG. 11 is a block diagram showing a structure of a signal processing device 16a as an example of the signal processing device 16. The signal processing device 16a includes a first frequency discriminator 101, a second frequency discriminator 102, a third frequency discriminator 103 and an arithmetic operation device 104.
The first frequency discriminator 101 conducts a frequency analysis upon receiving a reception signal from the first photo detector 27 and extracts a Doppler frequency from a target. The second frequency discriminator 102 conducts the frequency analysis of the monitor signal and obtains the frequency difference fM between the laser pulse 21 and the local light 25 and the oscillation timing of the laser pulse from the monitor signal. The third frequency discriminator 103 transmits the error signal based on the value of the frequency difference fM between the laser pulse 21 and the local light 25 to the control circuit 18 from the monitor signal. The arithmetic operation device 104 calculates a distance and a velocity of the target on the basis of an output signal from the first and second frequency discriminators 101 and 102.
In this example, the structure of the frequency discriminator that functions as the first frequency discriminator 101, the second frequency discriminator 102 and the third frequency discriminator 103 includes an A/D converter that converts the reception signal or the monitor signal into a digital signal and a signal processing portion that processes the digital signal converted by the A/D converter into a necessary signal by a frequency analyzing means such as a fast Fourier transform (FFT) as shown in FIG. 12.
Also, as shown in FIG. 13, the frequency discriminator may be made up of an electric filter portion which is made up of one or a plurality of electric filters, and a signal processing portion that conducts a necessary signal processing in accordance with the transmittance of a signal from the electric filter portion.
Also, the signal processing device 16 can employ a signal processing device 16b that incorporates the function of the third frequency discriminator 103 shown in FIG. 11 into the second frequency discriminator 102 shown in FIG. 11, as shown in FIG. 14, and has the same function as that of the signal processing device 16a shown in FIG. 11.
As described above, in the photo detecting portion 10 of the coherent laser radar device using the conventional injection seeding pulsed laser 4 as the light source, the second photo detector 28 for monitoring the oscillation frequency of the injection seeding pulsed laser 4 is disposed in addition to the first photo detector 27 that detects the reception light as shown in the photo detecting portion 10a shown in FIG. 10. In addition, at least two systems for the reception signal, the monitor and so on are prepared for the frequency discriminator as shown in FIGS. 11 and 14.
In the case where the intensity of the monitor light is sufficiently large, a part or the entire microwave amplifier of the second photo detector 28 can be omitted.
Subsequently, an influence of an internal reflection light of the coherent laser radar device using a pulsed laser light source and a coaxial transmission/reception optical system as shown in FIG. 9 will be described.
In FIG. 9, the beam splitter 5, the xc2xc wavelength plate 6, the telescope 7 and the scanning optical system 8 are of the coaxial transmission/reception optical system that makes the optical axes 22 of the transmit/reception lights substantially coincide with each other.
In the coherent laser radar device using the coaxial transmission/reception optical system of this type, the internal reflection lights from the optical elements that constitute the coaxial transmission/reception optical system reach the photo detecting portion 10 through the same path as that through which the reception light passes. In particular, since the internal reflection lights from the telescope 7 and the scanning optical system 8 pass through the beam splitter 5 as a reception side, the influence of the internal reflection light is large. As usual, the attenuation of reflection of the telescope 7 and the scanning optical system 8 is about 60 to 70 dB. On the contrary, the attenuation of reflection of the reception light from aerosol contained in the atmosphere exceeds 100 dB.
In order to conduct a high-precision measurement, the microwave amplifier which is made up of the pre-amplifier and the post-amplifier of the first photo detector 27 within the photo detecting portion 10a shown in FIG. 10 is required to amplify the reception signal up to about a degree suitable for the maximum sampling amplitude of the first frequency discriminator 101. Since the reception light is slight, the microwave amplifier of the first photo detector 27 has a high gain. Since the internal reflection light is much larger in power than the reception light, the internal reflection light induces the saturation of the microwave amplifier in the first photo detector 27. Since the linear amplification of the signal is not conducted until the microwave amplifier is restored since the microwave amplifier is saturated, the measurement cannot be conducted. As usual, it takes several xcexcs until the influence of such an internal reflection light is eliminated. For that reason, a xe2x80x9cblind zonexe2x80x9d where the short distance of several hundreds of m from the device cannot be measured occurs.
As described above, the coherent laser radar device using the conventional injection seeding pulsed laser 4 shown in FIG. 9 as the light source and also using the coaxial transmission/reception optical system suffers from the following drawbacks.
1. In order to monitor the oscillation frequency of the injection seeding pulsed laser 4, the photo detector for monitoring is disposed in addition to the photo detector that detects the reception light, resulting in the complicated photo detecting portion.
2. Likewise, at least two systems for the reception signal and the monitor are disposed for the frequency discriminator with the result that the signal processing device is complicated.
3. The wide xe2x80x9cblind zonexe2x80x9d where the measurement cannot be conducted over the short distance of several hundreds of m from the device occurs due to the influence of the internal reflection light of the transmission/reception optical system.
The present invention has been made to eliminate the above-described problems, and therefore an object of the present invention is to provide a coherent laser radar device using an injection seeding pulsed laser as a light source and also using a coaxial transmission/reception optical system in which a photo detector is of one system and the blind zone can be eliminated.
In order to achieve the above-mentioned object, according to the present invention, there is provided a coherent laser radar device, comprising: a local light source that oscillates at a single frequency; a pulsed laser that oscillates at a frequency which is identical with or close to an output light of the local light source as a single frequency; a transmission/reception optical system that irradiates a pulsed laser beam from the pulsed laser toward a target as a transmission light and receives a scattered light from the target as a reception light; a light coupling means that couples the output light from the local light source and the reception light; a photo detecting portion that conducts light coherent detection on the light coupled by the light coupling means; and a signal processing device that calculates a speed and a distance of a target in accordance with an output from the photo detecting portion, characterized in that the photo detecting portion comprises: a photo detecting element that conducts the light coherent detection; a microwave switch that changes over a propagation path of an output from the photo detecting element; a microwave amplifier; and a switch control means that changes over the microwave switch so as to transmit a signal before a reference time to the signal processing device as a monitor signal and transmit a signal after the reference time to the signal processing device as a reception signal with a time at which the pulse light from the pulsed laser has completely passed through the transmission/reception optical system as the reference time.
Also, the pulsed laser includes an adjusting mechanism that adjusts a cavity length, the device further comprises a control circuit that controls the adjusting mechanism, and the control circuit outputs to the adjusting mechanism a control signal that adjusts the cavity length of the pulsed laser on the basis of an error signal from the signal processing device based on a frequency difference between the laser pulse and the local light.
Also, the microwave amplifier is made up of a pre-amplifier that amplifies a signal from the photo detecting element and a post-amplifier that amplifies an output of the pre-amplifier, and the microwave switch is disposed between the pre-amplifier and the post-amplifier, outputs a signal that has been amplified by the pre-amplifier as a monitor signal and outputs a signal that has passed through the post-amplifier as a reception signal.
Also, the microwave amplifier is made up of a pre-amplifier that amplifies a signal from the photo detecting element and a post-amplifier that amplifies an output of the pre-amplifier, and the microwave switch is disposed between the photo detecting element and the pre-amplifier, outputs a signal from the photo detecting element as a monitor signal and outputs a signal that has passed through the post-amplifier as a reception signal.
Also, according to another aspect of the present invention, there is provided a coherent laser radar device, comprising: a local light source that oscillates at a single frequency; a pulsed laser that oscillates at a frequency which is identical with or close to an output light of the local light source as a single frequency; a transmission/reception optical system that irradiates a pulsed laser beam from the pulsed laser toward a target as a transmission light and receives a scattered light from the target as a reception light; a light coupling means that couples the output light from the local light source and the reception light; a photo detecting portion that conducts light coherent detection on the light coupled by the light coupling means; and a signal processing device that detects a speed and a distance of a target in accordance with an output from the photo detecting portion, characterized in that the photo detecting portion comprises: a photo detecting element that conducts the light coherent detection; a microwave amplifying portion that amplifies an output signal from the photo detecting element; and a gain control means that controls the gain of the microwave amplifying portion so that an amplitude of the output signal from the microwave amplifying portion does not exceed a given threshold value.
Also, the pulsed laser includes an adjusting mechanism that adjusts a cavity length; the device further comprises a control circuit that controls the adjusting mechanism, and the control circuit outputs to the adjusting mechanism a control signal that adjusts the cavity length of the pulsed laser on the basis of an error signal from the signal processing device based on a frequency difference between the laser pulse and the local light.
Also, the microwave amplifier is made up of a pre-amplifier that amplifies a signal from the photo detecting element and a gain control amplifier that amplifies an output of the pre-amplifier, and the gain control means controls the gain of the gain control amplifier.
Also, the microwave amplifier is made up of a pre-amplifier that amplifies a signal from the photo detecting element and a post-amplifier that amplifies an output of the pre-amplifier, and the gain control means comprises a microwave variable attenuator disposed between the pre-amplifier and the post-amplifier, and an attenuation control circuit that controls the attenuation of the microwave variable attenuator.
Also, a coherent laser radar device according to still another aspect of the present invention is characterized by comprising: a local light source that oscillates at a single frequency; a pulsed laser that oscillates at a frequency which is identical with or close to an output light of the local light source as a single frequency; a transmission/reception optical system that irradiates a pulsed laser beam from the pulsed laser toward a target as a transmission light and receives a scattered light from the target as a reception light; a light coupling means that couples the output light from the local light source and the reception light; a photo detecting portion that conducts light coherent detection on the light coupled by the optical coupler; and a signal processing device that detects a speed and a distance of a target in accordance with an output from the photo detecting portion; a light variable attenuator disposed between the transmission/reception optical system and the photo detecting portion; and a control means that controls the attenuation of the light variable attenuator in such a manner that an amplitude of the output from the photo detecting portion does not exceed a given threshold value.
Further, the pulsed laser includes an adjusting mechanism that adjusts a cavity length, the device further comprises a control circuit that controls the adjusting mechanism, and the control circuit outputs to the adjusting mechanism a control signal that adjusts the cavity length of the pulsed laser on the basis of an error signal from the signal processing device based on a frequency difference between the laser pulse and the local light.